Wastewater treatment is the largest biotechnology industry in the world. This industry handles and disposes domestic and industrial wastes so they present no threat to the population and the environment.
The treatment of sewage or wastewater is most commonly first done in primary clarification (settling) tanks where the solids which settle are removed before the partially treated wastewater is then fed for secondary treatment in a biological treatment plant, where microorganisms degrade and stabilize the organic wastewater to biomass (sludge), water and gases.
The microorganisms that grow on the substrate in the wastewater are separated from the water by further settling of the “reacted” wastewater in the biological tanks, which has carbon substrates measured as biochemical oxygen demand (BOD) or chemical oxygen demand (COD), leaving a relatively clean effluent as the treated effluent. The treated effluent will then be discharged into open waters or send for further tertiary treatment or for reuse. This biological treatment is by, far the most common treatment process for municipal and industrial wastewaters.
Most biological treatment plants now use the conventional activated sludge process (CAS). CAS has proved useful for the treatment of many organic wastes which were at one time thought to be toxic to biological systems. This process is a treatment technique in which wastewater and reused biological sludge full of living microorganisms is mixed and aerated. The mixture formed of wastewater and biological sludge is designated as mixed liquor. After the mixed liquor has been formed in the aeration tank of an activated sludge process (CAS), excess mixed liquor is discharged into settling tanks and the treated supernatant is run off to undergo further treatment before discharge. Part of the settled material, the sludge, is returned to the head of the aeration system to re-seed the new sewage or wastewater entering the tank. “Excess sludge” which eventually accumulates beyond what is returned is removed from the treatment process to keep the ratio of biomass feeded to sewage or wastewater (F/M ratio) in balance.
However, the activated sludge process has a large footprint due to the need for large aeration tanks and clarifier tanks. The consistency of the effluent quality will be dependent on the bioprocess especially sludge bulking which caused serious problems to the operation of the activated sludge process (CAS) and greatly reduced the quality of the effluent.
In conventional activated sludge (CAS) systems, biomass separation from the biologically treated effluent relies on gravitational settling of aggregated mixed microbial flocs. Good separation is highly dependent on the flocculating characteristics of the mixed bacteria population. As the settling abilities of the biomass are relatively poorer at high biomass concentrations, the biomass concentrations in the CAS treatment process are usually limited to 5 g/l. In addition, the CAS wastewater treatment process also generates large quantities of excess sludge due to a shorter sludge retention time (SRT), normally in the region of 5-15 days. The treatment and disposal of this “excess sludge” usually represent 50-60% of the total cost for CAS wastewater treatment plant (Egemen, E., Corpening, J., Nirmalakhandan, N., 2001, Water Sci. Technol., vol. 44(2-3), p. 445-452). These drawbacks can be largely circumvented if the biomass is completely or almost completely retained. This leads researchers into the study of more efficient and cost effective wastewater treatment techniques in recent years.
A membrane bioreactor or submerged membrane biological reactor system (MBR) has been paid increasing attention in recent years for its advantages over the CAS wastewater treatment. By replacing the settling tank of a CAS with a membrane filtration device in an MBR, the biomass concentration can be increased, leading to high organic loading, compact model structure system and low sludge production, i.e. also a lower amount of excess sludge which would need to be deposited (Huang, X., Gui, P., Qian, Y., 2001, Process Biochemistry, vol. 36, p. 1001-1006). An MBR uses microporous or ultraporous membrane module(s) to separate the sludge from the activated sludge process (CAS). Therefore, clean water permeates from the one side of the membrane to the other side of the membrane and suspended solids (SS), bacteria and viruses are retained in the bioreactor membrane tank of the MBR.
Furthermore, in an MBR, the high level of mixed liquor suspended solids (MLSS) effectively achieves nitrification and denitrification without the need for extended aeration. Another advantage of the use of membrane filtrations is the indifference to hydraulic fluctuations. Theoretically, a MBR runs at a very low F/M ratio and is supposed to achieve a low sludge production. Applying the MBR with no sludge discharge in industrial application, however, is still not possible.
Thus, to avoid unnecessary sludge production, it is important to further improve the existing systems for cleaning sewage or wastewater.